Liquids flowing through nanoscale channels can slip; that is, there is a discontinuity in the mean speed between the walls and the first layer of liquid molecules. The mechanisms of slip are unclear. Using numerical simulation, we find an exponential dependence of slip on solvation pressure which can be explained by treating slip as a rate process. Predictions for the temperature and viscosity dependencies of slip agree with published data. Our findings are consistent with a description of slip as due to the propagation of molecular-size vacancies along the solid-liquid interface.